1. Field of the Invention
This invention relates to a technique for performing a self-test on integrated circuits that include an analog to digital converter.
2. Description of the Prior Art
The testing of integrated circuits that include analog to digital converters typically involves supplying an analog test signal from an external source, and monitoring the digital outputs. The test signal is typically a ramp voltage, and the digital output codes may be stored in a cache memory for later analysis. This analysis usually checks to determine that all the desired digital codes are produced, and that the codes increase monotonically. Checks for linearity are also included in some cases. For example, one type of all codes test uses an off-chip "match register" that is loaded sequentially with all possible codes. A ramp voltage is supplied to the analog input, and the match register is monitored by software to ensure that all the codes are achieved by the end of the ramp. Otherwise, a time-out signal causes a failure to be indicated. However, the conventional testing techniques not only require a significant amount of test equipment, but also may require probing a wafer with a multiplicity of probes to gain access to the various signal points of a given integrated circuit.
The subject of built-in self test (BIST) for integrated circuits has received considerable attention in recent years. The BIST concept generally provides for including in an integrated circuit at least some of the circuitry required to perform tests to ensure its proper operation. For example, the test signals may be generated on-chip, and some or all of the circuitry required to analyze the results may be integrated into the chip under test. The goal is typically to simplify testing during manufacture, by reducing the number of test signals that must be supplied to the IC, and the amount of test equipment required to analyze the results. In addition, system diagnostic tests in the field after manufacture are much easier to accomplish, and even real-time failure diagnostics may be performed.
The output of a BIST circuit may, if desired, be a simple flag set to indicate either a pass or fail condition for the IC, or a portion thereof. For example, a technique to test IC memory arrays is described in U.S. Pat. No. 4,872,168 coassigned herewith. If desired, the output pass/fail flag may be provided to the IC package terminals according to the Joint Test Action Group specifications, (IEEE Stantard 1149.1) in order to implement boundary level scanning techniques. That is, each integrated circuit on a board may be queried to perform a self-test cycle, whereby all of the circuitry on the board may be tested. FIG. 4 illustrates one possible configuration to test a multiplicity of such IC's on a circuit board using the JTAG Test Access Port (TAP). In FIG. 4, TCK, TMS, TDI, and TDO denote the test clock, test select, test data-in and test data-out signals on the TAP, respectively. Input pin TMS is used to activate the self-test on integrated circuits IC1-IC4. The test clock and the test data are applied through the input pins TCK and TDI, respectively. The test outcome is stored in a register on the circuit board and is scanned out through the output pin TDO.
However, while BIST techniques for digital circuits are becoming well established, there has been comparatively very little BIST development for the analog portions of circuits that include both analog and digital circuitry. Such mixed-signal integrated circuits often contain hidden nodes that are difficult to observe and test. Since it is the nature of analog circuits to be susceptible to drift and other variances, the need for BIST for operational checks in analog circuits is perhaps even greater than for digital circuits. For example, in telephonic systems, avionics, machine controllers, process controllers, and automotive systems, to name only a few, it is common to perform analog-to-digital (A/D) conversion. It is essential to ensure the accuracy of the A/D conversion, to help ensure the accuracy of other operations that are subsequently performed digitally. Therefore, there is a significant need to provide BIST techniques for integrated circuits that include analog-to-digital converters.